Let go of a stone and a feather at the same moment. The stone drops straight down; the feather drifts, wobbles, and lands long after. What made the two falls so different?
In earlier grades you saw that gravity acts on every object, toward Earth. Something besides gravity touched the feather: the drag of moving through air. That drag is called air resistance.
Watch five falls, one at a time. Each drawing marks every influence that matters to the motion.
When gravity is the only influence acting on an object's motion — with air resistance too small to matter — the motion is called free fall.
So the stone, the slipping coin, and the tossed ball are in free fall. The drifting feather is not: the air drags on it strongly enough to steer its motion. The ramp-riding coin is not either: the ramp acts on it too, so gravity is not acting alone.
Free fall doesn't require being dropped — thrown up, thrown down, or released, what matters is what acts during the motion.
Which of these objects is in free fall?
Try the rule on two fresh cases. An acorn breaks off a branch and falls, with air resistance too small to matter. Gravity is the only influence acting on the acorn's motion, so the acorn is in free fall.
Now a soap bubble drifts slowly toward the ground. The air steers the bubble strongly enough to change its motion. Gravity is not the only influence, so the bubble is not in free fall.
A skydiver descends with her parachute open. Why is she NOT in free fall?
Which of these objects is in free fall?
Your turn
A ball is thrown straight down from a bridge and has just left the thrower's hand. Air resistance is too small to matter. Is the ball in free fall?
A pebble is falling in free fall. Which change would take its motion OUT of free fall?
A leaf flutters side to side as it drifts down from a branch. Why does its fall NOT count as free fall?